The present invention relates to a document feeder for a copier, facsimile apparatus, scanner or similar image forming apparatus and, more particularly, to a document feeder capable of reducing the first copy time.
A document feeder is usually mounted on an image forming apparatus in order to convey a document, so that image information can be read out of the document. The document feeder includes a table to be loaded with a stack of documents, a document conveying section, and a document discharging section. The document conveying section conveys documents stacked on the table one by one to a glass platen mounted on the body of the apparatus, and then conveys each document read to the discharging section.
Japanese Patent Laid-Open Publication No. 9-166831, for example, discloses a document feeder capable of raising a table loaded with documents at the time of document feed and thereby allowing the documents to be fed by a conveying section one by one from the table. In this document feeder, as soon as the trailing edge of the last document in the direction of document feed moves away from feeding means, the document raising member is lowered. The problem with such a construction is that a document feeding operation is not allowed to start until the document raising member started to move upward reaches a preselected document feed position. The document feeder therefore increases the first copy time, i.e., a period of time necessary for the first copy to be output.
To reduce the first copy time, the document raising member may be raised as soon as document sensing means senses the documents set on the table. This, however, brings about another drawback that defective document feed is apt to occur when, e.g., the operator fails to accurately set the documents. Moreover, it is difficult for the operator to add other documents after the document raising member has been raised.
Assume that the power switch of the document feeder is turned off while document feed is under way. Then, the document raising member stops at its raised position, depending on a mechanism used to drive it. This makes it difficult for a person who turns on the power switch later to set documents unless the person performs an extra operation of lowering the document raising member. Also, if the document raising member is held in its raised position at the time of a jam, then a jamming document cannot be easily removed.
SADF (Semi Automatic Document Feeder) mode is available with the conventional document feeder for automatically feeding a document every time it is set on the table by hand. The problem with the conventional SADF mode is that when the document raising member is lowered in response to the output of the document sensing means indicative of the absence of documents, a period of time necessary for the up-down movement of the member is increased, slowing down the copying operation.
Furthermore, when thin or otherwise soft documents are fed, the last document or lower documents hang down at their leading edges in the direction of document feed. Such documents are therefore apt to fold or to jam the transport path due to defective feed. This is also true when the documents stacked on the table are curled downward (back curl).
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-101460, 8-268571, and 9-175667.